For at least six centuries, residents along a lake in the mountains of central Japan have marked the depth of winter by celebrating the return of a natural phenomenon once revered as the trail of a wandering god.

It would only appear after days of frigid temperatures had frozen Lake Suwa into a sheet of solid white. First, people were awakened at night by a loud rumbling. Dawn broke to reveal its source: a long, narrow ridge of jagged ice that had mysteriously arisen across the lake’s surface, meandering like the spiked back of a twisting dragon.

This was the Miwatari, meaning the sacred crossing, which local belief held was left by a passing god of Japan’s native Shinto belief. Its appearance evoked feelings of awe but also reassurance among the residents, who ventured onto the ice to perform a ceremony honoring what they saw as a visitation from the supernatural. In the rare winters when the ice ridge did not appear, the god’s absence was viewed as a warning that the natural world was out of balance.

So important was the Miwatari that residents recorded whether it appeared, the condition of the lake and what historical events accompanied it. They have loyally written these descriptions every winter since 1443, creating a remarkable archive that attests to centuries of monotonously cold winters.